Butterfly valves are widely used in processing plants for controlling the flow of liquids and gases. Butterfly valves generally comprise a circular disc, which in closed position extends across the flow passage. In open position, turned through 90.degree. with the edge of the disc exposed to the stream, the butterfly valve allows a smooth, almost unimpeded flow of fluid. However, since the valve disc remains in the line, it is not possible to pass a cleaning device or "pig", as is commonly employed to scour and clean the internal walls of a pipeline.
Some butterfly valves employ resilient elastomer sleeves or seal rings to seal around the circumference of the valve disc. Such seals are, however, subject to wear by caustic or abrasive liquids that are commonly used in processing plants to clean the pipe system.